1. Technical Field
The present invention relates to a method for producing a thin film, and more particularly, to a method for forming a thin film on a substrate.
2. Description of Related Art
Silicon on insulator (SOI) is made essentially by providing an insulating layer beneath a silicon wafer, so as to avoid undesired electric effects, reduce power consumption and thereby lower current loss. Besides, SOI can increase the processing speed of an integrated circuit (IC). In addition to being applied to devices requiring lower power consumption, such as cell phones and watches, SOI is also finding applications in high frequency ICs to make the best use of its high-speed processing features.
SOI can be produced by several different methods, as described below in detail. In 1988, Dr. W. Maszara, who was based in the United States, used an etch stop layer to produce bond and etch-back silicon on insulator (BESOI) having a micron-scale film thickness. However, as the working time for the etch stop layer to stop etching varies at different points on a wafer, a thin film formed on the BESOI may exhibit undesirable total thickness variation (TTV). Moreover, BESOI involves a time-consuming manufacturing process that ends up with environment-unfriendly waste solutions, and in consequence the production cost of BESOI stays high.
About the same time, International Business Machines Corp. (IBM) produced SOI using a method called Separation by Implantation of Oxygen (SIMOX). Since the SOI produced by SIMOX showed better total thickness variation, BESOI almost became obsolete in applications related to highly integrated circuits.
In 1992, Dr. M. Bruel, who was based in France, invented a method for producing a thin film, better known as the Smart Cut® Process. With the Smart Cut® Process, a thin film having a sub-micron-scale film thickness can be successfully cut off from a substrate and transferred to another substrate. In addition, the Smart Cut® Process allows a thin film on BESOI to have a total thickness variation as low as a thin film made by SIMOX.
U.S. Pat. No. 5,374,564 discloses a method for producing a thin semiconductor material film. According to the method, a high dose of ions, such as gas ions of hydrogen or an inert gas, are implanted into a primary substrate to form an ion layer. Then, the primary substrate is bonded with a target substrate to form a single piece. A heating treatment follows, causing the gas ions in the ion layer to polymerize and generate numerous microbubbles. The microbubbles gradually unite into a whole and thereby partially separate the primary substrate into two layers. One of the separated layers of the primary substrate is transferred to the target substrate and thereby forms a thin film on the target substrate. The Smart Cut® Process not only produces a thin film having a low total thickness variation and a low defect density, but also generates no corrosive solution during the manufacturing process. Besides, gas dissipated from the Smart Cut® Process is neither toxic nor harmful, so there are no pollution problems. Moreover, the primary substrate can be recycled for further use.
In the Smart Cut® Process as well as in SIMOX, the thickness of a thin film on SOI is determined by an ion implantation depth, which, in turn, is controlled by an ion implantation energy. Since the Smart Cut® Process uses hydrogen ions, which have very light mass, the ion implantation depth cannot be easily reduced to nanoscale even with a lower ion implantation energy. As a result, it is difficult to achieve shallow implantation with a good ion distribution and produce a thin film of a uniform thickness. Furthermore, after a thin film is transferred to a target substrate using the Smart Cut® Process, it is still necessary to apply chemical polishing or oxide etching to decrease the thickness of the thin film to nanoscale. Thus, when it comes to a wafer having a large area, the accuracy and uniformity of thickness of a transferred thin film could be significantly impaired.
As for SIMOX, although oxygen ion implantation is used, wherein oxygen ions have heavier mass than hydrogen ions, and shallow implantation with a good ion distribution is achievable, so that an ultra-thin SOI film can be produced, the yield of components may be lowered by defects resulting from SIMOX, particularly oxygen precipitates generated by oxygen ion implantation, as semiconductor manufacturing processes enter the nanoscale era.
In order to achieve a desirable nanoscale thickness with the Smart Cut® Process, U.S. Pat. No. 5,882,987 discloses a method for producing a thin semiconductor film using the Smart Cut® Process, wherein an etch stop layer is grown on a primary silicon substrate, before an ultra-thin monocrystalline silicon layer is grown on the etch stop layer. Then, the Smart Cut® Process is employed to cut off a combined portion comprising the ultra-thin monocrystalline silicon layer, the etch stop layer and superfluous silicon beneath the etching stop layer from the primary silicon substrate, and transfer the combined portion to a target substrate.
Following that, a surface of the target substrate is etched so as to remove the superfluous silicon from the etch stop layer, so that all that remain on the target substrate are the ultra-thin monocrystalline silicon layer and the etch stop layer. After applying the method for making BESOI, an ultra-thin SOI wafer is finally produced, though unfortunately with a non-uniform film thickness.
In summary, while the Smart Cut® Process can be used to produce SOI, the lighter mass of hydrogen ions makes it difficult to achieve shallow implantation with a good ion distribution. Consequently, the transferred thin film is less likely to have a desired nanoscale film thickness. If the transferred thin film is to have a nanoscale film thickness, an additional thinning step is required, which, however, may considerably lower the accuracy and uniformity of the film thickness. Moreover, since a matching level of crystal lattices between the etch stop layer and the thin film is likely to affect film quality, product yield may drop as a result.